The invention relates to a 12-phase static electrical transformer rectifier.
An electrical transformer is an electrical machine having a primary circuit and a secondary circuit. A transformer serves to modify the voltage and current values of the electricity as delivered by an alternating current (AC) source of electrical energy to the primary circuit of the transformer.
Conventionally on-board an aircraft, when there is a need to convert a three-phase AC voltage into a direct current (DC) voltage while providing isolation, use is made of a 12-phase static transformer rectifier, also known as a 12-pulse transformer rectifier unit (TRU).
In a static transformer, energy is transferred from the primary circuit to the secondary circuit via a magnetic circuit that is constituted by the body of the transformer. These two circuits are then magnetically coupled together, which serves to provide isolation between the two circuits.
As shown in FIG. 1, the 12-phase transformer rectifiers 100 in general use are constituted by a transformer 40 followed by two six-phase rectifiers 50 and 60, each formed by a bridge of six diodes 70 and coupled to a load 3 via two interphase inductors C. The transformer 40 comprises a primary circuit 110 having three primary coils 120 in a star configuration and coupled to the power supply network 2, and two secondary circuits 111 and 112.
The first secondary circuit 111 comprises three first secondary coils 130 in a star configuration with each of the free terminals 130b of the coils 130 coupled to a respective branch 50a, 50b, 50c of the first three-phase rectifiers 50.
The second secondary circuit 112 comprises three second secondary coils 140 in a delta configuration, with each of the three nodes 140b of the delta configuration being connected to a respective branch 60b of the second three-phase rectifier 60.
This topology corresponds to the network quality requirements expected for electricity networks in aviation.
In such a circuit, the phase-to-neutral secondary voltages of the transformer are phase shifted by 30°, i.e. by π/6. Because of this 30° phase shift between the phase-to-neutral secondary voltages, there is a phase shift of 30° between the two secondary line currents.
If the transformation ratio between the star primary winding and the star secondary is written m′, and the transformation ratio between the star primary winding and the delta secondary is written m, then it can be shown that there exists a simple relationship between m and m′ for the circuit to operate, namely: m=√{square root over (3)}m′.
The current flowing in the secondary windings can be determined simply. For the delta secondary, there exists a relationship between of the secondary line currents and the winding currents. This relationship is valid over time and it is possible to write:
         {                                                                      J                                  2                  ⁢                  a                                            ⁡                              (                t                )                                      =                                                            I                                      2                    ⁢                    a                                                  ⁡                                  (                  t                  )                                            -                                                I                                      2                    ⁢                    c                                                  ⁡                                  (                  t                  )                                                                                                                                    J                                  2                  ⁢                  b                                            ⁡                              (                t                )                                      =                                                            I                                      2                    ⁢                    b                                                  ⁡                                  (                  t                  )                                            -                                                I                                      2                    ⁢                    a                                                  ⁡                                  (                  t                  )                                                                                                                                    J                                  2                  ⁢                  c                                            ⁡                              (                t                )                                      =                                                            I                                      2                    ⁢                    c                                                  ⁡                                  (                  t                  )                                            -                                                I                                      2                    ⁢                    b                                                  ⁡                                  (                  t                  )                                                                        where J2a, J2b, J2c are the currents flowing respectively in the three second secondary coils 140 in a delta configuration, and I2a, I2b, I2c are the currents respectively delivered to each of the three branches 60a, 60b, 60c of the second rectifier circuit 60 to which the second secondary coils 140 in a delta configuration are coupled, as shown in FIG. 1.
The 12-phase transformer rectifier converts the three-phase AC voltages, e.g. having a root mean square (rms) voltage of 115 volts (V) or of 230 V, into a DC voltage of 28 V or of some other voltage, for example. This transformation is performed by the transformer and the two three-phase rectifiers.
At present there does not exist a solution based on a 12-phase transformer rectifier that is simple and reliable and that enables the quality of the electricity network to be improved still more effectively.